526 research outputs found
A special irreducible matrix representation of the real Clifford algebra C(3,1)
4x4 Dirac (gamma) matrices (irreducible matrix representations of the
Clifford algebras C(3,1), C(1,3), C(4,0)) are an essential part of many
calculations in quantum physics. Although the final physical results do not
depend on the applied representation of the Dirac matrices (e.g. due to the
invariance of traces of products of Dirac matrices), the appropriate choice of
the representation used may facilitate the analysis. The present paper
introduces a particularly symmetric real representation of 4x4 Dirac matrices
(Majorana representation) which may prove useful in the future. As a byproduct,
a compact formula for (transformed) Pauli matrices is found. The consideration
is based on the role played by isoclinic 2-planes in the geometry of the real
Clifford algebra C(3,0) which provide an invariant geometric frame for it. It
can be generalized to larger Clifford algebras.Comment: 23 pages LaTeX, to appear in the J. Math. Phys. (v2: appendix B on
Pauli matrices and references are added, minor other changes
Surprises in the doping dependence of the Fermi surface in Bi(Pb)-2212
A detailed and systematic ARPES investigation of the doping-dependence of the
normal state Fermi surface (FS) of modulation-free (Pb,Bi)-2212 is presented.
The FS does not change in topology away from hole-like at any stage. The data
reveal, in addition, a number of surprises. Firstly the FS area does not follow
the usual curve describing Tc vs x for the hole doped cuprates, but is
down-shifted in doping by ca. 0.05 holes per Cu site, indicating either the
break-down of Luttinger's theorem or the consequences of a significant bi-layer
splitting of the FS. Secondly, the strong k-dependence of the FS width is shown
to be doping independent. Finally, the relative strength of the shadow FS has a
doping dependence mirroring that of Tc.Comment: 5 pages, 4 figures (revtex
Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus
We show that the optical and electronic properties of nanocrystalline silicon
can be efficiently tuned using impurity doping. In particular, we give
evidence, by means of ab-initio calculations, that by properly controlling the
doping with either one or two atomic species, a significant modification of
both the absorption and the emission of light can be achieved. We have
considered impurities, either boron or phosphorous (doping) or both (codoping),
located at different substitutional sites of silicon nanocrystals with size
ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped
nanocrystals have the lowest impurity formation energies when the two
impurities occupy nearest neighbor sites near the surface. In addition, such
systems present band-edge states localized on the impurities giving rise to a
red-shift of the absorption thresholds with respect to that of undoped
nanocrystals. Our detailed theoretical analysis shows that the creation of an
electron-hole pair due to light absorption determines a geometry distortion
that in turn results in a Stokes shift between adsorption and emission spectra.
In order to give a deeper insight in this effect, in one case we have
calculated the absorption and emission spectra going beyond the single-particle
approach showing the important role played by many-body effects. The entire set
of results we have collected in this work give a strong indication that with
the doping it is possible to tune the optical properties of silicon
nanocrystals.Comment: 14 pages 19 figure
An ARPES view on the high-Tc problem: phonons vs spin-fluctuations
We review the search for a mediator of high-Tc superconductivity focusing on
ARPES experiment. In case of HTSC cuprates, we summarize and discuss a
consistent view of electronic interactions that provides natural explanation of
both the origin of the pseudogap state and the mechanism for high temperature
superconductivity. Within this scenario, the spin-fluctuations play a decisive
role in formation of the fermionic excitation spectrum in the normal state and
are sufficient to explain the high transition temperatures to the
superconducting state while the pseudogap phenomenon is a consequence of a
Peierls-type intrinsic instability of electronic system to formation of an
incommensurate density wave. On the other hand, a similar analysis being
applied to the iron pnictides reveals especially strong electron-phonon
coupling that suggests important role of phonons for high-Tc superconductivity
in pnictides.Comment: A summary of the ARPES part of the Research Unit FOR538,
http://for538.wmi.badw.d
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